The field of this invention is instrumentation for use in nuclear physics and nuclear medicine, and, more particularly, means for collimating X and gamma radiation.
It is well known in the prior art to provide a collimator for X or Gamma radiation which is fabricated from an assembly of lead (or other high-Z metal, Z being the conventional symbol for atomic number, as indicated, for example, in Goodwin, Quimby, and Morgan, "Physical Foundations of Radiology," Harper & Row (4th Ed. 1970), p. 18) strips arranged in a corrugated configuration with passageways or channels ranging in cross-section from a few millimeters to a few centimeters. In such collimators, the interchannel septa (i.e., the lead strips) may be on the order of one millimeter in thickness and a few millimeters in width. The collimator may alternatively have an "egg-box" configuration with interlocking septa providing rectangular cross-section channels.
The limiting spatial resolution of such collimators is set by the channel diameters, and so it is desirable to make these as small as possible. Resolution is also limited by the solid angles defined by the channel entrances and exits. Further, the septa must be of sufficient depth in the direction of propagation and of sufficient thickness transverse to the direction of propagation so that substantially all the uncollimated radiation which enters the entrance face of the collimator is absorbed before reaching the exit face. Finally, the proportion of properly collimated radiation that will actually pass through the collimator depends upon the relationship between the aggregate open area of the channels to the aggregate frontal area of the walls or septa which divide the channels from one another. Therefore it is desirable to make the septa as thin as possible. Because the absorption coefficient of the septal material rises very rapidly with atomic number (Z), the septa are normally fabricated from lead or some other strongly absorbing, high atomic number (high-Z), material. Lead is most often used because of its relatively low cost, although the softness of lead places substantial limits on the minimum septa thickness.
An alternate collimation technique uses a lead block with an array of circular cross-section channels drilled therein. However, the prior art collimators of these types have been limited to channels having approximately 10 square millimeter cross-sections with 0.5 millimeter inter-channel spacing. Due to the softness of lead, higher channel density results in collapse of inter-channel walls.
As a result of recent investigations in the subject of X-ray collimation techniques, an assembly of glass channel mosaics has been considered as still another alternative form of X-ray collimator. Such channel mosaics have been previously used in electron-multipliers for image tubes. In that field, the electron-multiplying glass commonly contains significant fractions of lead oxide. In addition, such glass mosaics can have channels on the order of a few microns in diameter. However, collimators which are manufactured of these lead-glass multiple channel mosaic assemblies are only effective in the collimation of low energy radiation having wavelengths greater than 1.0 Angstrom, primarily because the proportion of lead by volume is only about 15% in glass formulations suited to the fabrication of channel mosaics. This limitation is especially significant in the field of nuclear medicine since the bulk of current diagnostic radiology requires collimated high energy radiation having a wavelength on the order of 0.15 Angstroms and smaller.
Accordingly, it is an object of the present invention to provide a high resolution collimator for X and gamma radiation, particularly for radiation less than 1.0 Angstrom in wavelength, hereinafter referred to as "hard radiation."
It is another object to provide a means for collimating radiation of wavelength 0.15 Angstrom and smaller.
Another object is to provide a method of fabrication of a high resolution X and gamma radiation collimator.